1. Field of the Invention
The present invention relates to a coordinates input device generally known as a touch panel, and more particularly to a coordinates input device for reading by converting a pressing position signal into an electric signal when the a surface having a planar resistive thin film thereon is pressed.
2. Description of the Related Art
An input device known as a touch panel is widely used recently in combination with a liquid crystal display device or the like. The touch panel is available in an analog input type and a digital input type, as shown in FIG. 1 and FIG. 2, respectively. In the coordinates input device of an analog input type, in FIG. 1, transparent conductive films 4, 5 such as ITO are formed on the entire confronting surfaces of a pair of transparent film substrates 2, 3, and electrodes 6, 7, 8, 9 of metal or other such conductors are formed at both ends in mutually orthogonal directions on the transparent conductive films 4, 5. The electrodes 6 to 9 are respectively connected to supply voltages V1 to V4 through switches S1 to S4. Such film substrates 2, 3 are mutually glued and fixed on a display device 10 such as a liquid crystal display device, and a coordinates input device 1 is thus composed.
In this coordinates input device 1, first of all, the switches S1, S2 are made to conduct, while the switches S3, S4 are cut off. As a result, a voltage (V1-V2) is applied between the electrodes 6, 7. When a pressing position P of the film substrate 2 is pressed in this state, an interior division voltage of the inter-electrode voltage (V1-V2) at the pressing position P by the resistance value of the transparent conductive film 4 is applied to the transparent conductive film 5, thereby turning on the switches S3, S4, and this voltage is read from the electrodes 8, 9. Consequently, the X-direction coordinate of the pressing position P is detected. As for the Y-direction coordinate of the pressing position P, to the contrary, by turning on the switches S3, S4, a voltage (V3-V4) is applied between the electrodes 8, 9, and the interior division voltage of the transparent conductive film 5 on the pressing position P is read from the electrodes 6, 7.
On the other hand, in the coordinates input device of the digital input type in FIG. 2, a pair of film substrates 2, 3 are prepared, and band-shaped transparent electrodes 12 extending in the X-direction are arranged in plural rows in the Y-direction on the film substrate 2. The transparent electrodes 12 are individually connected to plural connection terminals 13 disposed at the end of the film substrate 2. On the other film substrate 3, transparent electrodes 14 extending in the Y-direction are disposed in plural rows in the X-direction, and the transparent electrodes 14 are individually connected to plural connection terminals 15 disposed at the end of the film substrate 3. By joining together the film substrates 2, 3, the crossing parts of the transparent electrodes 12, 14 are arranged in a matrix, and they are fixed on the display device 10. Here, when the pressing position P is pressed, the transparent electrodes 12, 14 are scanned in time division by a control device such as a microcomputer to which the connection terminals 13, 15 are connected, and the transparent electrodes 12, 14 corresponding to the pressing position P are detected, and the X-coordinate and Y-coordinate in the unit of the transparent electrodes 12, 14 at the pressing position P are detected.
In the coordinates input device 1, an input is made by pressing the film substrate 2 with a tapered pen member. At this time, the hand holding the pen member is often used on the film substrate 2 and, in order to avoid entry of wrong input by the pressing member from a low pressing pressure (e.g., by a hand), a granular spacer made of electric insulating resin material or the like is disposed between the film substrates 2, 3 at a specific height and at a relatively high density. In such a coordinates input device 1, it is impossible to enter data by using the human finger or the like, as the pressing member thus the utility is low.
In contrast, in the coordinates input device 11 shown in FIG. 2, although a digital coordinates input is possible by using the transparent electrodes 12, 14 extending in mutually orthogonal directions, an analog input of a continum of coordinates as in FIG. 1 is impossible, and thus the utility is low.
FIG. 16 is a block diagram of an input and output device as applied in the prior art and in an embodiment of the invention.
In FIG. 16, the input and output device comprises a CPU (central processing unit) 71 for managing and controlling the device centrally, ROM 72 and RAM 73 for storing a program and data, a clock 74, an I/O (input/output) unit 75 including a printer and CRT (cathode-ray tube) connected to the device, a keyboard 76 for entering data to the device from outside, a display unit 77 including a display such as a liquid crystal display, a display drive circuit 78 for driving the display unit 77 controlled by the CPU 71, an A/D converter (analog/digital converter) 79, a tablet unit 80, and a tablet controller 81 for driving and controlling the tablet unit 80 controlled by the CPU 71.
FIG. 17 shows the construction of the tablet unit 80 given in FIG. 16. The tablet unit 80 is composed by combining an X-direction resistive film 80a which is a resistive thin film having electrodes disposed at both ends in the horizontal direction in the drawing (that is, the X-direction), and a Y-direction resistive film 80b which is a resistive thin film having electrodes disposed at both ends in the vertical direction in the drawing (that is, the Y-direction), through a uniform tiny clearance. In the X-direction resistive film 80a, when a voltage is applied from the tablet controller 81, electricity flows from the X-HIGH side electrode to the X-LOW side electrode. In the Y-direction resistive film 80b, when a voltage is applied from the tablet controller 81, electricity flows from the Y-HIGH side electrode to the X-LOW side electrode.
In this operation, the user presses a desired position on the surface of the tablet unit 80 by using the tip of pen, pencil or the like so as to make the resistive films 80a and 80b contact each other at the contact pressing position. This contact pressing force is detected as an electric signal, and when this signal reaches a specific level, voltage division values on the resistive films 80a and 80b, (which are proportional to, the pressing position) are detected as voltage signals. These voltage signals are individually led out by the X-direction resistive film 80a and Y-direction resistive film 80b, and are input to the CPU 71 through the A/D converter 79. The CPU 71 drives the display drive circuit 78 depending on the digital input values, and the circuit 78 displays and drives the corresponding XY coordinate liquid crystal picture element on the display unit 77. In this way, when the user presses a desired position on the tablet unit 80, the picture element of the display unit 77 corresponding to the pressing position is displayed and driven.
The conventional opera%ion for input of coordinates by the input and output device shown in FIG. 16 is explained below by reference to FIG. 3, according to the processing flow in FIG. 4.
The CPU 71 repeatedly executes the processing flow for measurement of coordinates shown in FIG. 4, according to the periodic interruption of a built-in timer.
The CPU 71 applies a voltage for measurement of the X-direction pressing, in the processing of step S100 in FIG. 4 (abbreviated as S100 in the diagram). In other words, the CPU 71 sets switches SW.sub.XH, SW.sub.XP shown in FIG. 3 in ON state through the tablet controller 81, and sets all other switches in OFF state. As a result, supply of voltage from a voltage source V through switch SW.sub.XH is started. The CPU 71 waits for a specific time until the supply voltage is stabilized, and then measures the pressing in the X-direction in the processing of next step S101.
Suppose the pressing position is a reference A in FIG. 3. At this time, the measured voltage V.sub.X is EQU V.sub.X =V.multidot.R.sub.LX /(R.sub.X1 +R.sub.P +R.sub.Y2 +R.sub.LX) (1)
The input resistance R.sub.ADX of the A/D converter is sufficiently large so as to not affect the measurement. Here, in where R.sub.P is the case of R.sub.X1, R.sub.Y2 &lt;&lt;R.sub.P (the contact resistance, which varies inversely with the pressing level), the measured voltage V.sub.X indicates the pressing level regardless of the pressing position.
In the processing steps S101 and S102, since the pressing level is proportional to the measured voltage V.sub.X, whether the pressing level is sufficient or not is judged on the basis of whether the voltage V.sub.X is larger than a specific value or not. At this time, if the pressing level is insufficient, measurement of coordinates is impossible, and processing ends.
When the pressing level is sufficient, processing following step S103 is executed.
In the processing at step S103, the CPU 71 applies a voltage for measurement of the X-direction coordinate. That is, the CPU 71 sets switches SW.sub.XH and SW.sub.XL shown in FIG. 3 in ON state through the tablet controller 81, and sets all other switches in OFF state. As a result, a voltage is applied in the X-direction of the resistive film 80a. The CPU 71 waits for a specific time until the applied voltage is stabilized, and then in the processing at step S104, the voltage V.sub.X is measured through the A/D converter 79. The A/D converter 79 converts the measured voltage V.sub.X into a digital signal, which is given to the CPU 71. Afterwards, the CPU 71 cancels the applied voltage to the tablet unit 80 through the tablet controller 81. That is, all switches shown in FIG. 3 are set in OFF state.
The CPU 71, in the processing at the next step S105, applies a voltage for measurement of the Y-direction coordinate. That is, the CPU 71 sets switches SW.sub.YH and SW.sub.YL shown in FIG. 3 in ON state through the tablet controller 81, and sets all other switches in OFF state. As a result, a voltage is applied to the resistive film 80b in the Y-direction from the voltage source V. The CPU 71 waits for a specific time until the applied voltage is stabilized, and in the processing at next step S106, the Y-direction coordinate is measured. The A/D converter 79 reads the measured voltage V.sub.Y, converts it into a corresponding digital signal, and inputs the signal to the CPU 71 for coordinate measurement.
After the coordinate measurement, at steps S107 to S109, pressing is measured again for confirmation. Thus, an intermediate release of the pen (lowering of pressing level) is detected.
In this procedure, after the pressing level is measured in the X-direction only, the coordinates of the pressing position A in the X-direction and the Y-direction are measured, and finally, for confirmation, the level in the X-direction pressing is measured again.
To measure the coordinates on the tablet unit 80 in this way, it is important that the pressing level at the measuring point is sufficient, and that the contact of the upper and lower resistive films, that is, the X-direction resistive film 80a and Y-direction resistive film 80b be secure. The contact resistance R.sub.P shown in FIG. 3 is inversely proportional to the pressing force, and if the pressing force is insufficient, the contact resistance R.sub.P increases, and measurement of coordinates becomes inaccurate. Or, if the contact of the resistive films 80a and 80b is insufficient, it is highly possible that the upper and lower resistive films depart from each other during the measurement of coordinates. It is also known that the contact resistance R.sub.P changes steeply with respect to change of pressing level. Therefore, for measurement of coordinates at high precision, it is important to measure the pressing level with high precision and confirm that a secure contact state is obtained.
Recently, however, owing to the requirements for larger size and smaller current consumption of the tablet unit 80, the relation is not always R.sub.X1, RY.sub.2 &lt;&lt;R.sub.P, and its effect cannot be ignored. That is, as shown in formula (1) above, for the measured voltage V.sub.X to indicate the pressing level regardless of the pressing position, a pressing level sufficient for measurement of a measured voltage V.sub.X is required. In other words, if the pressing force is insufficient due to the pressed position of the measured value of the pressing level are large, and this the measurement of coordinates itself may be inaccurate.